Convert to Sass

This version moves the primary source code from CSSNext to Sass (.scss) in order to allow splitting it into multiple even smaller modules. By using @include in your main SCSS file, you can choose each module invididually from the files and folders in src/modules. This change should allow people to customize their styling even further to only use the pieces of lighter and fuel that then need, resulting in an even smaller CSS output!

This also fixes an issue that prevented <code> blocks from displaying correctly.